1. Field of the Invention
The present invention relates to an engine starting apparatus for starting an engine by driving to rotate the engine with a starter motor, and more particularly to a so-called solenoid driven type engine starting apparatus for starting an engine when a drive gear connected to a starter motor is moved to a driven gear of the engine for mesh engagement with the driven gear.
2. Description of the Related Art
Conventionally, an engine starting apparatus of the above-mentioned type is known as disclosed in Japanese Patent Unexamined Publication Hei.9-310667. This engine starting apparatus includes a starter motor, a planetary gear speed reduction mechanism connected to the starter motor, a pinion gear movably provided on an output shaft of the planetary gear speed reduction mechanism for mesh engagement with a ring gear provided on a crankshaft of the engine, a one-way clutch provided integrally on the pinion gear, and a magnet switch for moving said pinion gear on the output shaft between a position where the pinion gear is brought into mesh engagement with the ring gear and a position where the pinion gear is brought out of mesh engagement with the ring gear. This one-way clutch is constructed so as to provide for power transmission between the output shaft and the pinion gear when the rotational speed of the pinion gear is smaller than the rotational speed of the output shaft of the planetary gear speed reduction mechanism and to cut off power transmission between the starter motor and the output shaft when the rotational speed of the pinion gear is greater than the rotational speed of the output shaft of the planetary gear speed reduction mechanism.
In this engine starting apparatus, when an engine starting operation through an ignition key is performed in an attempt to start the engine, the starter motor starts to rotate, and the rotation of the starter motor is transmitted to the output shaft by the planetary gear speed reduction mechanism at reduced speeds, whereby the pinion gear is rotated. At the same time, the magnet switch moves the pinion gear together with the one-way clutch to the ring gear side for mesh engagement with the ring gear, whereby the rotational force of the starter motor is transmitted to the engine via the planetary gear speed reduction mechanism, the output shaft, the one-way clutch, the pinion gear and the ring gear to start the engine. Then, after the engine has been started, the magnet switch moves the pinion gear in a direction in which the pinion gear moves apart from the ring gear for release the mesh engagement between the pinion gear and the ring gear. As this occurs, when the rotational speed of the pinion gear driven by the engine comes to increase over the rotational speed of the output shaft of the planetary gear speed reduction mechanism which is driven by the starter motor after the engine has been started, the rotational force of the engine is not transmitted to the starter motor side by the operation of the one-way clutch, whereby the forced excessive rotation of the starter motor by the engine is avoided.
According to the aforesaid conventional engine starting apparatus, since the one-way clutch needs to be moved together with the pinion gear when the engine is attempted to be started, as this actually occurs, the resulting inertia mass becomes equal to the sum of the masses of those two components, and this causes problems that louder impact noise tends to be generated when the pinion gear is brought into mesh engagement with the ring gear and hence that the durability of those components tends to be deteriorated. In addition, the driving force needed to drive the one-way clutch as well as the pinion gear increases for the same reason, and this increases in turn the size of the magnet switch and requires a longer time before the pinion gear and the one-way clutch actually start to move, leading to the slow response of those components when they need to operated. Furthermore, since the construction requires the one-way clutch and the planetary gear speed reduction mechanism, the casing of the apparatus has to be enlarged for accommodation of those components therein. In addition, since the speed reduction is implemented through meshing of the gears in the planetary gear speed reduction mechanism, there is caused a problem that the operation noise gets relatively louder.
The invention was made with a view to solving the problems, and an object thereof is to provide an engine starting apparatus which can attain entirely reduction of noise, miniaturization of the engine starting apparatus and improvement in durability thereof.
With a view to attaining the object, according to a first aspect of the invention, there is provided an engine starting apparatus 1 comprising a driven gear (for example, a ring gear 23 in an embodiment (referred to as the same in this first aspect)) connected to an engine, a starter motor 3 for starting the engine which is adapted to be driven to rotate when the engine is started, a speed-reduction and power-cut-off mechanism (a wedge roller type speed reduction mechanism 10) connected to the starter motor 3 and having an rotatable output shaft 19, which speed-reduction and power-cut-off mechanism is adapted to output a rotation of the starter motor 3 from the output shaft 19 at a reduced speed and to cut off power transmission between the starter motor 3 and the output shaft 19 when the rotational speed of the output shaft 19 exceeds the rotational speed thereof resulting after the speed reduction is implemented for the output of rotations of the starter motor 3, a drive gear (a pinion gear 4) provided on the output shaft 19 in such a manner as to rotate together with the output shaft 19 and to move axially over the output shaft 19 between an engagement position (a position shown in FIG. 4B) where the drive gear is brought into mesh engagement with the driven gear (the ring gear 23) and a disengagement position (a position shown in FIG. 4A) where the drive gear is brought out of engagement with the driven gear (the ring gear 23), and a driving unit (a magnet switch 5) for driving the drive gear (the pinion gear) to the engagement position when the engine is started and to the disengagement position after the engine has been started.
According to the engine starting apparatus, when the engine is started, the starter motor is driven to rotate, and the rotation of the starter motor is transmitted to the drive gear via the output shaft at the reduced speed by the speed-reduction and power-cut-off mechanism. In addition, the drive gear is driven to the engagement position by the driving unit for mesh engagement with the driven gear, whereby the power of the starter motor is transmitted to the engine via the drive gear and the driven gear which is in mesh engagement with the drive gear to start the engine. After the engine has been started, the drive gear is driven to the disengagement position, whereby the mesh engagement between the drive gear and the driven gear is cancelled. As this occurs, since the power transmission between the output shaft and the starter motor is prevented when the rotational speed of the output shaft exceeds the rotational speed thereof resulting after the rotational speed of the starter motor is reduced by the speed-reduction and power-cut-off mechanism, the forced excessive rotation of the starter motor by the engine is avoided after the engine has been started. Namely, since the speed-reduction and power-cut-off mechanism functions as a one-way clutch, the mechanism is different from the conventional example in that there is no need to provide a one-way clutch together with drive gear, whereby the inertia mass of the drive gear can be reduced by that extent, thereby making it possible to reduce the impact noise generated when the drive gear is brought into mesh engagement with the driven gear and to improve the durability of the gears. In addition, the driving unit for driving the drive gear can be miniaturized for the same reason, this leading to the miniaturization of the whole of the apparatus in the end. In addition, since the drive gear is allowed to move quickly, the response when the engine is started can be improved.
According to a second aspect of the invention, there is provided an engine starting apparatus 1 as set forth in the first aspect of the invention, wherein the speed-reduction and power-cut-off mechanism is constituted by a wedge roller type speed reduction mechanism 10.
According to this engine starting apparatus, since the wedge roller type speed reduction mechanism is employed, the engine starting apparatus of the invention is different from the conventional planetary gear speed reduction mechanism in that noise can be reduced by such an extent that there is produced no mesh engagement of gears.